Air pollution

Environmental StudiesUnit:3

Definition :

• “Air pollution is the presence in ambient atmosphere of substances, generally resulting from the activity of man in sufficient concentration, present for a sufficient time and causes the harmful effect on human, plant and animal”

Sources of Air pollution

Sources of Air pollution

It is mainly classified in two

Natural

Man made/Anthropogenic

Natural sources• Pollen grains

• Volcanic eruption

• Forest fire

• Salty spray from ocean

• Dust storms

• Marshy land

• Bacteria and other micro organisms

• Spores

• Photochemical reaction Forest fire

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Dust storms

Salty spray from oceanMarshy land

Volcanic eruption

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Man made sources

• In contrast to the natural sources of air pollution man made sources pose problems severe enough to endanger life and property

It includes :• Industrial units

• Thermal power plant

• Automobile exhaust

• Fissile fuel burning

• Agricultural activities

• Mining

• Aircraft

• Nuclear explosion

• Domestic burning of woods

Man made sources generally classified into following categories:

Point sources/stationary sources : Industrial units

Line /mobile sources : Automobiles

Area sources : Town & Cities

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Classification of Air pollutants

Air pollutants

Based on origin Based on state of matter

Primary Eg.SOX,NOX,Ash,smoke

Secondary Eg.O3,SO3,PAN etc.

Gaseous Eg.CO,NOX,SOX etc.

Particulates Eg.Dust , Smoke, aerosol, soot,Fly ash etc.

a. Based on origin of air pollutants Primary pollutants

Secondary pollutants

Primary pollutants :which are emitted directly from identifiable sources and are found in the

atmospheres in the form in which they were emitted.

• Sulphur oxide

• Nitrogen oxides

• Hydrocarbons

• Ash

• Smoke

• Dust

• Fumes

• Mist

• Radioactive compounds

Secondary Pollutants

These are formed in the atmosphere by chemical interactions between primary pollutants and atmospheric constituents.

• Ozone

• Sulphur trioxide

• Paroxyacyl nitrate (PAN)

• Aldehydes

• Ketons etc

According to the state of matter ,Air pollutants are classified as

1. Gaseous pollutants :These pollutants are in gaseous state at normal temperature and pressure.

• CO

• CO2

• NOX

• SOX

• Hydrocarbons

• Photochemical oxidants

2. Particulate air pollutants :These includes Suspended droplets ,Solid particles /there mixture in the

Atmosphere

For example

Aerosols(All air suspension smaller than 1mm)

• Dust

• Smoke

• Fumes

• Mist

• Fog

• Fly ash

• Soot

Natural particulates such as

• Pollen grains

• Protozoa

• Fungal spores

• Volcanic dust

Aerosols

These includes all air borne suspensions of solid or liquid particles smaller than

1nm.

Eg. Dust, smoke, mist, fumes

Dust : it consist of small solid particles (size 1-200 µm), which are formed by the breaking up of larger masses of rocks and soil either by natural disintegration or by mechanical processes of crushing, grinding or blasting etc.

They remain suspended in air temporarily but ultimately settle under the influence of gravity

Smoke :it consist of fine solid particles (size 0.1 – 1 µm) resulting from the incomplete combustion of organic particles like coal, wood ,tobacco or other chemical processes.

Depending upon the nature of the matter burnt, smoke may have different odours .

Fumes : These are fine solid particles (size 0.1-1 µm ) formed by the condensation of vapours of solid materials .

They are odourless vapours which may as may not be visible and are usually released from chemical as metallurgical processes.

Mist : It consist of liquid droplets (size smaller than 10 µm )formed by the condensation of vapours in the atmosphere or are released from industrial operations.(such as formation of sulphuric acid mist.

Fog : If the mist is made up of water droplets whose concentration is high and dense enough to obscure vision then the mist is called fog.

Fly ash : This consist of finely divided non combustible particles present in the gases arising from fuel combustion.

It contains in organics metallic or mineral substances released when the organic part of coal is burnt.

Soot :Incomplete combustion of carbon containing materials release carbon particles.

A collection of such particles is soot.

Air Pollution Effects

Table 1: Sources, Health and Welfare Effects for Criteria Pollutants.

Pollutant Description Sources Health Effects Welfare Effects

Carbon

Monoxide

(CO)

Colorless, odorless

gas

Motor vehicle exhaust,

indoor sources include

kerosene or wood burning

stoves.

Headaches, reduced mental

alertness, heart attack,

cardiovascular diseases,

impaired fetal development,

death.

Contribute to the formation of

smog.

Sulfur Dioxide

(SO2)

Colorless gas that

dissolves in water

vapor to form acid,

and interact with other

gases and particles in

the air.

Coal-fired power plants,

petroleum refineries,

manufacture of sulfuric acid

and smelting of ores

containing sulfur.

Eye irritation, wheezing, chest

tightness, shortness of breath,

lung damage.

Contribute to the formation of

acid rain, visibility impairment,

plant and water damage,

aesthetic damage.

Nitrogen

Dioxide (NO2)

Reddish brown, highly

reactive gas.

Motor vehicles, electric

utilities, and other

industrial, commercial, and

residential sources that

burn fuels.

Susceptibility to respiratory

infections, irritation of the lung

and respiratory symptoms

(e.g., cough, chest pain,

difficulty breathing).

Contribute to the formation of

smog, acid rain, water quality

deterioration, global warming,

and visibility impairment.

Ozone (O3) Gaseous pollutant

when it is formed in

the troposphere.

Vehicle exhaust and certain

other fumes. Formed from

other air pollutants in the

presence of sunlight.

Eye and throat irritation,

coughing, respiratory tract

problems, asthma, lung

damage.

Plant and ecosystem damage.

Lead (Pb) Metallic element Metal refineries, lead

smelters, battery

manufacturers, iron and

steel producers.

Anemia, high blood pressure,

brain and kidney damage,

neurological disorders,

cancer, lowered IQ.

Affects animals and plants,

affects aquatic ecosystems.

Particulate

Matter (PM)

Very small particles of

soot, dust, or other

matter, including tiny

droplets of liquids.

Diesel engines, power

plants, industries,

windblown dust, wood

stoves.

Eye irritation, asthma,

bronchitis, lung damage,

cancer, heavy metal

poisoning, cardiovascular

effects.

Visibility impairment,

atmospheric deposition,

aesthetic damage.

Pollutants Sources Effects on Vegetables

Aldehydes Photochemical reactions The upper portions of Alfalfa etc. will be affected to Narcosis if

250 ppm of aldehydes is present for 2 hrs duration.

Ozone (O3) Photochemical reaction of hydrocarbon

and nitrogen oxides from fuel

combustion, refuse burning, and

evaporation from petroleum products.

All ages of tobacco leaves, beans, grapes, pine, pumpkins and

potato are affected. Fleck, stipple, bleaching, bleached

spotting, pigmentation, growth suppression, and early

abscission are the effects.

Peroxy Acetyl

Nitrate (PAN)

The sources of PAN are the same as

ozone

Young spongy cells of plants are affected if 0.01 ppm of PAN

is present in the ambient air for more than 6 hrs.

Nitrogen dioxide

(NO2)

High temperature combustion of coal, oil,

gas, and gasoline in power plants and

internal combustion engines.

Irregular, white or brown collapsed lesion on intercostals

tissue and near leaf margin. Suppressed growth is observed

in many plants.

Ammonia & Sulfur

dioxide

Thermal power plants, oil and petroleum

refineries.

Bleached spots, bleached areas between veins, bleached

margins, chlorosis, growth suppression, early abscission, and

reduction in yield and tissue collapse occur.

Chlorine (Cl2) Leaks in chlorine storage tanks,

hydrochloric acid mists.

If 0.10 ppm is present for at least 2 hrs, the epidermis and

mesophyll of plants will be affected.

Hydrogen fluoride,

Silicon

tetrafluoride

Phosphate rock processing, aluminum

industry, and ceramic works and

fiberglass manufacturing.

Epidermis and mesophyll of grapes, large seed fruits, pines

and fluorosis in animals occur if 0.001 ppm of HF is present

for 5 weeks.

Pesticides &

Herbicides

Agricultural operations Defoliation, dwarfing, curling, twisting, growth reduction and

killing of plants may occur.

Particulates Cement industries, thermal power plants,

blasting, crushing and processing

industries.

Affects quality of plants, reduces vigor & hardness and

interferences with photosynthesis due to plugging leaf

stomata and blocking of light.

Mercury (Hg) Processing of mercury containing ores,

burning of coal and oil.

Greenhouse crops, and floral parts of all vegetations are

affected; abscission and growth reduction occur in most of

the plants.

Table-2: Sources, Effects of Air Pollutants on Vegetables

Air pollution control:Principles of controls, source control

Source Control Technology

Air quality management sets the tools to control airpollutant emissions.

Control measurements describes the equipment,processes or actions used to reduce air pollution.

The extent of pollution reduction varies amongtechnologies and measures.

The selection of control technologies depends onenvironmental, engineering, economic factors andpollutant type.

Settling Chambers

• Settling chambers use the force of gravity to remove solidparticles.

• The gas stream enters a chamber where the velocity of the gas isreduced. Large particles drop out of the gas and are recollectedin hoppers. Because settling chambers are effective in removingonly larger particles, they are used in conjunction with a moreefficient control device.

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Cyclones

• The dry chamber can be converted into wet cyclone scrubber by inserting high pressure spray.

• Centrifugal force

Figure: Cyclone Collector 5

Fabric Filters

• Fabric filters, or baghouses, remove dust from agas stream by passing the stream through aporous fabric. The fabric filter is efficient atremoving fine particles and can exceedefficiencies of 99 percent in most applications.

Figure: Fabric filter (baghouse) components 6

Conti…..

The selection of the fiber material and fabricconstruction is important to baghouseperformance.

The fiber material from which the fabric ismade must have adequate strengthcharacteristics at the maximum gastemperature expected and adequate chemicalcompatibility with both the gas and thecollected dust.

One disadvantage of the fabric filter is thathigh-temperature gases often have to becooled before contacting the filter medium.

Electrostatic Precipitators (ESPs)

• An ESP is a particlecontrol device that useselectrical forces to movethe particles out of theflowing gas stream andonto collector plates.

• The ESP places electricalcharges on the particles,causing them to beattracted to oppositelycharged metal plateslocated in theprecipitator.

Figure: Electrostatic precipitator components 7

Conti….

• The particles are removed from the plates by "rapping"and collected in a hopper located below the unit.

• The removal efficiencies for ESPs are highly variable;however, for very small particles alone, the removalefficiency is about 99 percent.

• Electrostatic precipitators are not only used in utilityapplications but also other industries (for other exhaustgas particles) such as cement (dust), pulp & paper (saltcake & lime dust), petrochemicals (sulfuric acid mist), andsteel (dust & fumes).

Control of gaseous pollutants from stationary sources

• The most common method for controllinggaseous pollutants is the addition of add-oncontrol devices to recover or destroy apollutant.

• There are four commonly used controltechnologies for gaseous pollutants:– Absorption,

– Adsorption,

– Condensation, and

– Incineration (combustion)

Absorption

The removal of one or more selectedcomponents from a gas mixture byabsorption is probably the mostimportant operation in the control ofgaseous pollutant emissions.

Absorption is a process in which agaseous pollutant is dissolved in aliquid.

Water is the most commonly usedabsorbent liquid.

As the gas stream passes throughthe liquid, the liquid absorbs the gas,in much the same way that sugar isabsorbed in a glass of water whenstirred.

Typical Packed Column Diagram

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Conti….

• Absorbers are often referred to as scrubbers, and there arevarious types of absorption equipment.

• The principal types of gas absorption equipment includespray towers, packed columns, spray chambers, and venturescrubbers.

• In general, absorbers can achieve removal efficiencies graterthan 95 percent. One potential problem with absorption isthe generation of waste-water, which converts an airpollution problem to a water pollution problem.

Adsorption

When a gas or vapor is brought into contact with a solid,part of it is taken up by the solid. The molecules thatdisappear from the gas either enter the inside of the solid,or remain on the outside attached to the surface. Theformer phenomenon is termed absorption (or dissolution)and the latter adsorption.

The most common industrial adsorbents are activatedcarbon, silica gel, and alumina, because they haveenormous surface areas per unit weight.

Activated carbon is the universal standard for purificationand removal of trace organic contaminants from liquid andvapor streams.

Carbon adsorption systems are either regenerative or non-regenerative.

Regenerative system usually contains more than one carbon bed. As one bed actively removes pollutants, another bed is being regenerated for future use.

Non-regenerative systems have thinner beds of activated carbon. In a non-regenerative adsorber, the spent carbon is disposed of when it becomes saturated with the pollutant.

Non-Regenerative Carbon Adsorption System

Regenerative Carbon Adsorption System

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Condensation

• Condensation is the process of converting agas or vapor to liquid. Any gas can be reducedto a liquid by lowering its temperature and/orincreasing its pressure.

• Condensers are typically used as pretreatmentdevices. They can be used ahead of absorbers,absorbers, and incinerators to reduce the totalgas volume to be treated by more expensivecontrol equipment. Condensers used forpollution control are contact condensers andsurface condensers.

• In a contact condenser, thegas comes into contact withcold liquid.

• In a surface condenser, thegas contacts a cooledsurface in which cooledliquid or gas is circulated,such as the outside of thetube.

• Removal efficiencies ofcondensers typically rangefrom 50 percent to morethan 95 percent, dependingon design and applications.

Surface condenser

Contact condenser

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Incineration

• Incineration, also known as combustion, is most used tocontrol the emissions of organic compounds from processindustries.

• This control technique refers to the rapid oxidation of asubstance through the combination of oxygen with acombustible material in the presence of heat.

• When combustion is complete, the gaseous stream isconverted to carbon dioxide and water vapor.

• Equipment used to control waste gases by combustion canbe divided in three categories:– Direct combustion or flaring,– Thermal incineration and– Catalytic incineration.

Direct combustor

• Direct combustor is a device in which air and allthe combustible waste gases react at the burner.Complete combustion must occurinstantaneously since there is no residencechamber.

• A flare can be used to control almost anyemission stream containing volatile organiccompounds. Studies conducted by EPA haveshown that the destruction efficiency of a flare isabout 98 percent.

In thermal incinerators the combustible waste gases passover or around a burner flame into a residence chamberwhere oxidation of the waste gases is completed.Thermal incinerators can destroy gaseous pollutants atefficiencies of greater than 99 percent when operatedcorrectly.

Thermal incinerator general case 11

Catalytic incinerators are very similar to thermal incinerators. Themain difference is that after passing through the flame area, thegases pass over a catalyst bed.

A catalyst promotes oxidation at lower temperatures, therebyreducing fuel costs. Destruction efficiencies greater than 95percent are possible using a catalytic incinerator.

Catalytic incinerator

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Book/Web Resources

• Environmental Pollution: Causes, Effects & Control by K.C Agrawal